1. Field of the Invention
This invention relates to an improved method of soldering and more particularly to an improved method of soldering simultaneously all of the connections of an assembly which include the pluarlity of electrical conductors disposed on a surface of electrically insulating material.
2. Description of the Prior Art
Although various types of printed circuits have been utilized in the past, one common type comprises a sheet of electrically insulating material, such as laminated sheets of paper impregnated with a synthetic resin and carrying on one surface of the sheet, one or more electrical conductors in the form of thin, flat strips integrally united to the insulation material. Where it is desired to mount a number of circuit components on the other side of the sheet of insulating material and connect them at many points to the printed conductors on the side previously mentioned, there is a considerable problem in making the connections rapidly and efficiently. In a typical assembly, over 100 connections may be involved, and to make each one of these connections individually with a soldering iron is a tedious process. Consequently, it is desirable to be able to use a process which will enable an operator to solder all of the connections in the same operation or operations. One method of soldering all such connections simultaneously is a dip-soldering technique. In this type of process, the entire side of the assembly containing the printed conductors with the leads from the circuit components projecting through the various points, can be dipped face down in a bath of molten solder and removed after a brief period of immersion. This results in coating the conductors with solder and soldering all of the connections at the same time.
An alternative method of soldering all such connections is the wave soldering technique. Originally developed to eliminate the deficiencies and limitations of the dip-soldering techniques, wave soldering involves pumping the molten solder through a nozzle to form a standing wave. In this type of process, the entire side of the assembly containing the printed conductors, with the leads from the circuit components projecting through the various points, travels at a predetermined rate of speed in contact with the surface of the wave of molten solder. To put it in other terms, the lower surface of the assembly will kiss the surface of the wave.
In such apparatus, solder is melted in a steel pot, generally at a temperature about 350.degree. to 650.degree. F. and preferably about 400.degree. to 550.degree. F. It is then pumped with a centrifugal pump up through the nozzle which may contain a series of internal baffles until a surface is formed at the top of the bath. This is referred to as the wave. The gently overflowing wave of liquid solder flows back into the solder pot reservoir from which it is recirculated by pumping up through the nozzle. A properly functioning solder wave contributes to the wetting of the joining surfaces, promotes through hole penetration, and helps assure formation of reliable solder joints and fillets. Wave soldering permits many solder connections of high reliability to be made in one pass of a printed circuit board through the system, makes possible careful regulation of time and temperature exposure, and assures that each connection receives virtually identical treatment. Also, the dynamic motion of the wave creates a washing or scrubbing action against the printed circuit board which aids wetting or spreading of the solder in the joint area.
A typical system includes stations for applying flux to the circuit, preheating the circuit board and performing the wave soldering operation. A conveyor is generally used to move the board through these stages. Precleaning and/or post-cleaning stations may be incorporated into the system, but these functions are usually performed by separate units of the production line, in line with or off line from the soldering system.
Dwell time in the solder is controlled by the part travel or conveyor speed, wave shape and configuration and wave dimension in the direction of travel. Machines with adjustable wave widths are available which are capable of providing extended dwell time for the work piece and the solder wave at high conveyance speeds. The speed of the operation must be coordinated with the minimum practical and most economical solder temperature, efficiency and productivity of the operation, with consideration for the rate at which a fast moving part can extract heat from the system. All this can be easily determined by those skilled in the art with a minimum of trial and error. The wave should be even and should contact the entire lower surface of the board as it passes over the wave. It should be smooth enough to prevent solder from passing over the top of even the thinnest boards.
The most commonly used wave probably is the bidirectional wave, formed by using a high capacity centrifugal pump that pumps the solder into a nozzle with a large plenum chamber. The solder pot must have sufficient area to make the wave formation insensitive to small variations in the level of solder in the pot. A stable wave of heights exceeding one inch (to handle long component leads) is achieved by pumping solder from the pot into the nozzle. The solder is pumped upward, rising through the nozzle until it overflows the sides of the nozzle, propelled with enough velocity to form a standing wave clear of the nozzle itself. Carefully placed baffle screens within the plenum of the nozzle can be used to shape and control the rise of the solder through the nozzle. Modern standing wave soldering systems are capable of pumping wave widths from 2 to 24 inches and wave heights up to 3 and 3/4 inch. They have relatively large solder capacities to insure consistency in soldering temperature and to compensate for heat loss and dissipation.
Oil intimately intermixed with the molten solder wave prevents dross formation on the wave, reduces the inherent surface tension of the solder, and the solder tends to spread more readily on clean conductor surfaces. The principal of the oil intermix is one in which oil is continuously fed to the input end of the solder pump. It is sucked in by the pump, intermixed with the solder and the mixture then driven to the surface. The oil dispersed in the solder then spreads out on the wave surface.
Cleaning is generally necessary after the soldering operation, particularly where oil is employed with the solder. Cleaning is most effective and quickly achieved immediately after the soldering operation while the boards are hot before the flux residues become hard. The liquid wave principle may also be employed with the cleaning compositions for the cleaning operation. Batch type cleaning using ultrasonic dip tanks and vapor degreasers as well as free-standing in line spray cleaning systems, may also be employed.
The liquid wave concept is a simple, practical and economic method of cleaning. The wave of cleaning solvent or liquid is pumped and formed in the same manner as a solder wave but with a far greater surface area. The height of the wave can be adjusted so that cleaning liquid just touches the bottom side of the printed circuit board assembly or is slightly submerged for a flushing action on the top side of the board as well.
The use of oil, intermixed with the molten solder requires solvent cleaning to remove the oil from the soldered assembly. Accordingly, it was discovered in the prior art that polyglycol-based fluids can be used in place of oil and that they permit wash-off with plain water rather than requiring a solvent for the cleaning. However, the prior art polyglycol-based fluids present serious problems of fuming at the elevated temperatures necessary for soldering.
Detailed discussions of wave soldering will be found in "Soldering Equipment, Wave or Cascade Type", pages 191 and 192 of the 1977 edition of Insulation Circuits Desk Manual; "Understanding the Solder Wave and Its Effects on Solder Joints, Insulation/Circuits", January 1978, pages 45 through 49 and "Guide to Wave Soldering Equipment, Insulation/Circuits", February 1977, pages 38 through 46.